This invention relates to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
An electrostatographic printing process forms an electrostatic latent image and reproduces the image in viewable form on a copy sheet. The field of electrostatography includes electrophotography and electrography. Electrophotography employs a photosensitive medium to form, with the aid of electromagnetic radiation, the electrostatic latent image. Electrography utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic latent image. In both of the foregoing processes, the latent image is rendered viewable by the process of development, i.e, depositing particles on the latent image. Frequently, the particles are then transferred from the latent image to a copy sheet, or, in some processes, the recording sheet on which the latent image is produced, may serve also as the copy sheet after the particles have been deposited thereon. In either case, the resultant toner powder image deposited on the sheet is permanently affixed thereto by the process of applying heat and/or pressure. In the illustrative embodiment hereinafter described, an electrophotographic printing machine will be employed.
In electrophotographic printing, the photoconductive member is charged to sensitize its surface. The charged photoconductive member is exposed to a light image of the original document being reproduced. Exposure of the sensitized photoconductive surface discharges the charge selectively in the irradiated areas, in accordance with the light intensity. This creates an electrostatic latent image on the photoconductive surface corresponding to the original document being reproduced. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing a developer mix into contact therewith. Typically, a developer mix comprises dyed or colored heat settable plastic powders, known in the art as toner particles, which are mixed with coarse carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. Thus, when the developer mix is brought into contact with the latent image recorded on the photoconductive surface, the greater attractive force thereof causes the toner particles to transfer from the carrier granules and adhere to the electrostatic latent image. This concept was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the art.
With the advent of multi-color electrophotographic printing, it became highly desirable to reproduce color originals as color copies or, even black and white originals as color copies. Heretofore, the process of color electrophotographic printing required the utilization of filters to form successive single color light images from the colored original document. These single color light images record successive single color electrostatic latent images on the photoconductive surface. Each single color electrostatic latent image is developed with toner particles of a color complementary to the color of the filtered light image. These toner powder images, each of a different color, are transferred to the copy sheet in superimposed registration with one another. Thereafter, the multi-layered toner powder image is permanently affixed to the copy sheet. This produces a multi-color copy from a colored original document. The foregoing is more fully described in U.S. Pat. No. 3,854,449 issued to Davidson in 1974.
Recently, it has become highly desirable to create flat color, i.e. a copy containing information in two or more colors. The foregoing may be achieved on a high speed electrophotographic printing machine by masking selected portions of the original document or utilizes two or more original documents and reproducing the information contained therein on a common copy sheet in different colors. Different colors in the electrophotographic printing machine may be achieved by changing the color of the toner particles employed in the development system. Thus, a black and white electrophotographic printing machine may be converted to a color printing machine simply by changing the color of the toner particles contained within the development system. This requires that the development housing be cleaned so as to have no residual toner particles therein prior to the introduction of the differently colored toner particles. This is necessary to prevent the contamination of the new charge of toner particles by the residual toner particles from the prior charge. For example, if the first charge of toner particles in the development system is black and the next charge red, the residual black toner particles would contaminate the red toner particles. It has been found that incomplete drainage of the developer housing of developer mix leaves a residual developer mix therein. This results in about a 2% to 3% contamination of the next charge of developer mix. When the developability of different species of toner particles on a carrier is about the same, the color difference caused by a 2% contamination results in borderline acceptability. However, where the first charge is dark and the second charge light, the contamination problem becomes more severe and 2% contamination level may not be acceptable.
Accordingly, it is a primary object of the present invention to improve purging of residual particles from the development system prior to the introduction of differently colored toner particles therein.